With various types of contact lenses including soft types and hard types provided from the past, there are cases when it is necessary to do circumferential direction positioning when wearing, for the contact lenses having an optical zone for astigmatism correction, a near/distance optical zone for correcting presbyopia and the like. For example, with a toric lens used for astigmatism correction, high level, stable matching is required for the relative position of the eyeball astigmatic axis and the lens cylindrical axis. Also, for example in a multifocal lens for both near and far vision used for correcting presbyopia or the like, with lenses using a lens design for which the lens power distribution is not uniform in the circumferential direction around the optical center and the like, it is also necessary to have circumferential direction positioning. Also, with this kind of contact lens, it is necessary to have both stability of the lens circumferential direction position and improved wearing comfort of the lens when wearing.
As methods for positioning in the circumferential direction with the contact lens in a worn state, from the past, the truncation method noted in Patent Document 1, the prism ballast method noted in Patent Document 2, and the slab-off method noted in Patent Documents 3 and 4 are known. However, with these conventional methods, it was extremely difficult to satisfy the required level about both the contact lens circumferential direction positioning performance and wearing comfort.
Specifically, with the truncation method noted in Patent Document 1, the lens lower edge outer periphery with a linear shape in the chord direction is supported on the lower eyelid, but there is strong irritation by both end edges of the linear shape on the lower eyelid, so it was difficult to obtain good wearing comfort. Also, the prism ballast method noted in Patent Document 2 uses the gravitational effect by a prism shape, but the lens lower edge which is thick irritates the lower eyelid, so it was difficult to obtain good wearing comfort. Furthermore, the slab-off method noted in Patent Documents 3 and 4 uses a tightly-holding effect of the eyelid in relation to a thin part provided at the lens lower part or the both upper and lower parts, but when trying to ensure the tightly-holding volume by the eyelid to ensure the circumferential direction position performance, the lens size becomes big, and there was the problem that wearing comfort was easily degraded.
Meanwhile, disclosed in Patent Document 5 is the peri-ballast method that stabilizes the lens circumferential direction position using the balance of lateral thick parts and the gravitational effect by displacing the lateral thick parts provided on the lens peripheral zone downward and setting the lens center-of-gravity downward. Also, for this peri-ballast method, in Patent Document 6, the applicant of the present invention disclosed a constitution that allows improvement of both the lens circumferential direction position stability and better wearing comfort by adjusting the thickness change rate in both sides of the circumferential direction with the lateral thick parts.
However, with the conventional peri-ballast method as noted in Patent Documents 5 and 6, the fact that there is still room for improvement regarding the circumferential direction positioning effect became clear by the inventors of the present invention. Specifically, by doing a great deal of experimenting and investigating, the inventors of the present invention found that with the conventional peri-ballast method, based on the balance effect of the lateral thick parts, the contact lens tends to stabilize even in a worn state with the contact lens inverted vertically. Thus, there is a new problem of cases of not returning to the normal state with top and bottom positioning correct.
It was also newly understood that depending on the eyelid shape, pressure, and the like of each wearer, there are cases when there is great fluctuation of the circumferential direction position in the worn state from the target normal circumferential direction position.
To deal with this kind of newly discovered problem, the inventors of the present invention also investigated improving performance for returning from the inverted state to the normal state and suppressing fluctuation from the normal position by the gravitational effect by setting the downward center-of-gravity displacement amount to be big using the lateral thick parts. However, when attempting to handle this simply by making the downward center-of-gravity displacement amount big using the lateral thick parts, the maximum thickness dimension of the lateral thick parts is set to be large. As a result, it is impossible to avoid the problem of a decrease in wearing comfort, so this was not really an effective measure for addressing this.